Thermoplastic olefins (TPOs) are uncrosslinked blends of olefin polymers, usually crystalline polyethylene or polypropylene, and olefin copolymer rubbers or elastomers, e.g., copolymers of olefins such as ethylene and propylene or butene. They can be made by physically blending in an internal mixer, or by polymerizing in a reactor. These materials have impact properties that allow them to be used in such applications as bumpers for automobiles and other automotive or industrial products. It is desirable under many circumstances to paint parts made from these TPOs. These materials are not paintable or coatable, because the paints or coatings consist of polar materials like urethanes, acrylics, epoxies, or melamines that have very poor adhesion to nonpolar materials like polyolefins. Various methods are used to enhance the paintability of the TPOs, e.g., flame treatment, surface oxidation, and plasma treatment, and in many cases an adhesion promoter is used as a tie layer between the TPO substrate and the paint coating. Such adhesion promoters usually contain chlorinated polyolefins and alkyd resins.
The rubber content and morphology of the thermoplastic olefin are important in obtaining good paint adhesion. In most cases the parts are injection molded and thus subjected to various shear forces that distort the elastomer particles. Two problems are encountered: (1) the paint adhesion is usually poor near high shear regions, e.g., near the gate area of the part, and (2) formulations that paint well have poor durability, i.e., the paint is removed due to scraping or shearing forces across the painted part during service, or by nicking due to stones or other debris. Hence there is a need for a thermoplastic olefin material that does not have these disadvantages.